To further troubleshoot:
Add something like 'auth.debug /var/log/auth.log' to your syslog
configuration.
Add 'log_level: 7' to /usr/lib/sasl2/slapd.conf
Run saslauthd in debugging mode, with the '-d' option.

With the same error or a different one?
If you're getting the same error, make sure that you have the SASL PLAIN
mechanism installed on your client system. Use pluginviewer/saslpluginvewer
to get a list. If you don't, you may need to install additional binary
packages provided by your OS vendor.
Using ldapwhomi is probably better at this point, and will help you work
out your authz-regexp statements.